Electrostatography broadly includes the forming and developing of electrostatic image patterns either with or without light exposure through the use of an electrostatically charged toner composition. It includes electrophotography, as used, for example, in office copying machines, as well as processes such as dielectric recording which require no light exposure.
A principal type of electrostatically chargeable toner composition for such processes is a dry fusible powder composed mainly of a thermoplastic binder resin and a colorant such as carbon black or an organic pigment or dye. This kind of dry toner powder is normally used in a developer composition in admixture with carrier particles (usually larger in size than the toner particles). These triboelectrically charge the toner particles so that they can then be electrostatically attracted to oppositely charged areas of an electrostatic latent image to develope the image, the toner thereafer being transferred to and fixed by fusion or other means to an image receiving sheet, e.g., a sheet of paper.
One successful type of dry toner composition employs as the binder resin a fusible styrene-acrylic copolymer which, advantageously, can be lightly cross-linked, as disclosed in the patent to Jadwin et al, Re. 31,072 of Nov. 2, 1982.
In the past, styrene-acrylic toners have been made by synthesizing the binder polymer in an emulsion polymerization or suspension polymerization reaction. The polymer is then melt blended with carbon black or other colorant and with a charge control agent and possibly other addenda. The blend is solidified and mechanically pulverized to small particle size. Styrene-acrylic toners made in this way have had at least one drawback--the polymer often contains a high concentration of surfactant or other materials required in the polymerization. These are difficult to remove by any economically feasible means. As a consequence the toners tend to be environmentally unstable. That is to say, their chargeability or ability to maintain a high level of electrostatic charge varies with the environmental conditions of temperature and humidity. As a consequence, low relative humidity can cause an increase in image mottle and low solid area density in the copies. High relative humidity results in increased image density and background development. Daily and seasonal fluctuations in the environment can thus result in unstable performance of the copying apparatus.
Vinyl polymers have also been made by a suspension polymerization method of the kind described in the patent to Wiley, U.S. Pat. No. 2,932,629. The patent, which did not relate to toner manufacture, called the method "quiescent suspension polymerization." According to Wiley, polymer beads of small and uniform size were obtained in a suspension polymerization process by dispersing a liquid vinyl monomer in an aqueous medium that contains a colloidal suspending agent such as bentonite clay. The latter functioned to limit the coalescence of the polymerizable monomer droplets, the result being small, uniform polymer beads.
Later publications have reported using a similar technique for synthesizing toner beads, with the colorant and other addenda being included in the reaction mixture. U.K. Patent Application 2,070,036A published Sep. 3, 1981 describes making a styrene-acrylic toner by suspension polymerization of the monomers in the presence of colloidal silica. By obtaining toner beads directly from the synthesis, grinding of the polymer is unnecessary.
Although the direct suspension polymerization process for toner beads avoids the problem of surfactant contamination of the toner, a serious problem occurs. The suspending agent or dispersant, as it is also called, remains firmly bound to the surfaces of the polymer beads in a high concentration. Unless removed in some way, this suspending agent will harm the electrical properties of the beads. The toner will not be chargeable to a high stable voltage. A possible explanation is that the suspending agent, e.g., hydrophilic silica, transfers from the toner particles to the carrier particles, coating the surfaces of the latter and preventing triboelectric charging of the toner. The problem evidently has been recognized in recent patents, though described in somewhat different terms. See, for example, Hyosu, U.S. Pat. No. 4,601,968 and Wada et al, U.S. Pat. No. 4,507,378. Both comment on attempts to wash a dispersant from the toner but, as the Hyosu patent indicates, too much of the dispersant will remain on the toner surfaces.
A hydrophilic polymeric suspending agent, namely, poly(vinyl alcohol), has also been used b applicants in combination with a polyester-promoted silica suspending agent in the suspension polymerization of certain vinyl monomers to form polymers from which blended toner compositions are made. The resulting toners, however, are less than desired with respect to environmental stability and fusing properties.
In summary, the preparation of polymers by conventional emulsion and suspension polymerization followed by melt compounding with addenda and pulverizing the blend has produced toners which contain undesirable contaminants. They are, therefore, electrically unstable with changes in the environmental humidity. On the other hand, the preparation of toner beads by suspension polymerization of monomers in admixture with toner addenda, yields toner beads without requiring pulverization. The toner surface, however, is contaminated with the suspending agent which prevents negative charging of the toner and can be removed only at considerable costs. Furthermore, polymers made by suspension polymerization using a promoted silica suspending agent which also includes a hydrophilic polymer suspending agent form blended toners which do not have the desired environmental stability.
In accordance with the present invention, toner particles are prepared which have excellent chargeabilty and environmental stability and excellent fusing properties. This is done without the need for a costly washing procedure.